Minimum Inhibitory Concentration Breakpoints and Disk Diffusion Inhibitory Zone Interpretive Criteria for Tilmicosin Susceptibility Testing against Pasteurella Multocida and Actinobacillus Pleuropneumoniae Associated with Porcine Respiratory Disease

Tilmicosin is a novel macrolide antibiotic developed for exclusive use in veterinary medicine. Tilmicosin has been approved as a feed premix to control porcine respiratory disease associated with Pasteurella multocida and Actinobacillus pleuropneumoniae. The development of antimicrobial susceptibility testing guidelines for tilmicosin was predicated on the relationship of clinical efficacy studies that demonstrated a favorable therapeutic outcome, on pharmacokinetic data, and on in vitro test data, as recommended by the National Committee for Clinical Laboratory Standards (NCCLS). The approved breakpoints for the minimum inhibitory concentration dilution testing for both species are resistant, ≥32 μg/ml, and susceptible, ≤16 μg/ml. The zone of inhibition interpretive criteria for disk diffusion testing with a 15-μg tilmicosin disk are resistant, ≤10 mm, and susceptible, ≥11mm.

Reversible monensin adaptation in Enterococcus faecium, Enterococcus faecalis and Clostridium perfringens of cattle origin: potential impact on human food safety

OBJECTIVES

To determine the stability/reversibility and mechanism of monensin adaptation in monensin-treated cattle isolates compared with reference bacterial isolates, exposed in vitro to high monensin concentrations.

METHODS

We evaluated the potential for cattle-origin strains of Clostridium perfringens, Enterococcus faecium and Enterococcus faecalis exposed to monensin in vivo (in vivo monensin-exposed isolates) to maintain or achieve the ability to grow in the presence of high monensin concentrations (in vitro monensin-adapted isolates). Twenty-one consecutive subcultures of the in vitro monensin-adapted strains were performed, and monensin MICs were determined for the 3rd, 7th, 14th and 21st subcultures (subcultured isolates). SDS-PAGE and transmission electron microscopy (TEM) were used to determine protein expression and visualize extracellular morphology changes.

RESULTS

Monensin-non-exposed isolates did not display monensin adaptation during in vitro monensin exposure. In contrast, in vivo monensin-exposed isolates displayed monensin adaptation enabling growth at 32× MIC. Upon consecutive subculturing, monensin MICs returned to baseline, or one dilution above, for the monensin-adapted strains. SDS-PAGE identified overexpression of a 14 kDa protein (C. perfringens) and a 20.5 kDa protein (E. faecium and E. faecalis) in the monensin-adapted isolates. TEM demonstrated that in vitro monensin-adapted strains had a significantly thicker cell wall or glycocalyx compared with in vivo monensin-exposed or subcultured isolates.

CONCLUSIONS

In vivo monensin-exposed isolates of C. perfringens, E. faecium and E. faecalis have the ability to grow in the presence of high monensin concentrations in vitro. This is associated with an increased thickening of the cell wall or glycocalyx that is reversible upon serial passage, suggesting a phenotypically expressed, but not genetically stable, trait.

Pan-European monitoring of susceptibility to human-use antimicrobial agents in enteric bacteria isolated from healthy food-producing animals

OBJECTIVES

To determine the antimicrobial susceptibility of Escherichia coli, Salmonella, Campylobacter and Enterococcus from cattle, pigs and chickens across the European Union (EU) using uniform methodology.

METHODS

Intestinal samples (1624) were taken at slaughter across five EU countries. Bacteria were isolated in national laboratories, whilst MICs were determined in a central laboratory for key antimicrobials used in human medicine. Clinical resistance was based on CLSI breakpoints and decreased susceptibility based on European Food Safety Authority (EFSA)/EUCAST epidemiological cut-off values.

RESULTS

Isolation rates were high for E. coli (n=1540), low for Salmonella (n=201) and intermediate for Campylobacter (n=940) and Enterococcus (n=786). For E. coli and Salmonella, clinical resistance to newer compounds (cefepime, cefotaxime and ciprofloxacin) was absent or low, but decreased susceptibility was apparent, particularly in chicken strains. Resistance to older compounds (except gentamicin) was variable and higher. Colistin resistance was absent for E. coli, but apparent for Salmonella. For Campylobacter jejuni, ciprofloxacin resistance was markedly prevalent for chickens, whereas clinical resistance and decreased susceptibility to erythromycin was absent or very low. For Campylobacter coli, resistance was notably higher. None of the Enterococcus faecium strains was resistant to linezolid, but some were resistant to ampicillin or vancomycin. Resistance to quinupristin/dalfopristin was frequent.

CONCLUSIONS

Resistance patterns varied widely depending on bacterial species, antibiotics, hosts and region. Resistance varied among countries, particularly for older antimicrobials, but clinical resistance to newer antibiotics used to treat foodborne disease in humans was generally very low. In the absence of resistance to newer compounds in E. coli and Salmonella, the apparent decreased susceptibility should be monitored.

Macrolide-resistant Campylobacter: the meat of the matter

The use of macrolide antibiotics in food animals has the potential to select for macrolide-resistant strains of resident bacterial flora. This may include the animal pathogens that are the intended targets of macrolide antibiotic intervention and Campylobacter, common inhabitants of the intestinal tract of food animals that are zoonotic pathogens in man. Such Campylobacter strains are not only resistant to the macrolide antibiotics used in food animals, e.g. tylosin, tilmicosin and tulathromycin, but to the macrolide antibiotics used in human medicine, e.g. erythromycin, azithromycin and clarithromycin, as well. Retail meat is a possible source of Campylobacter and persons consuming the meat derived from macrolide-treated food animals could acquire infections due to macrolide-resistant strains of this organism. Erythromycin is sometimes used to treat human cases of campylobacteriosis and those infected with animal-derived macrolide-resistant Campylobacter may not respond to treatment. The actual risk to human health from the use of macrolide antibiotics in food animals has been difficult to determine because of a lack of information about the macrolide-resistant Campylobacter found on the farm and in the clinic. Recently, however, a plethora of new information has become available on this topic. This review discusses what is currently known about the selection of macrolide-resistant Campylobacter in food animals, the prevalence of macrolide-resistant Campylobacter on retail meat, the prevalence of animal-derived macrolide-resistant Campylobacter in the clinic and the human health consequences associated with macrolide-resistant Campylobacter infection. This work will emphasize the comprehensive body of data generated in Denmark and the US as part of government-sponsored research studies over the last 10 years. These scientific findings may allow informed decisions to be made in the future about how macrolide antibiotics should be used in food animals while still safeguarding human health.

Animal health pharmaceutical industry

The animal health pharmaceutical industry has proactively reported on the volumes of member company antimicrobial active ingredients sold in the U.S. At the individual company level, reporting of finished product distribution data to the FDA is a regulatory requirement, with applications to surveillance and pharmacovigilance. An accounting of product manufactured is done for purposes of good business practices, as well as marketing analyses. Additional applications of antimicrobial usage data might include use in risk assessments, such as for the FDA's Center for Veterinary Medicine Guidance for Industry #152 for the evaluation of the microbiological safety of antimicrobials intended for use in food animals. Compilation of national usage data will be a complex undertaking, hindered by issues such as confidentiality, auditing, field use practice variations, population dynamics (e.g. disease incidence, market conditions for poultry and livestock production), and generic usage. The amounts or volumes in pounds should be considered relative to the large number of animals under husbandry in the United States. Large volumes might seem impressive unless put into proper context. Until such time as a clearly defined application of national usage data is agreed, it is recommended that local usage programs will provide more useful information to perpetuate prudent antimicrobial use in animals.

Prevalence of a novel capsule-associated lipoprotein among pasteurellaceae pathogenic in animals

Capsular serotype A strains of Pasteurella multocida of avian origin express a 40-kDa lipoprotein (Plp-40) thought to attach the extracellular polysaccharide to the cell surface. The objective of the present study was to assess the prevalence of Plp-40 in P. multocida strains of disparate serotypes and host origins, as well as other pathogenic members of the family Pasteurellaceae. Exponential-phase reference and clinical isolates were radiolabeled with [3H]-palmitate, lysed to obtain whole-cell protein fractions, and analyzed using SDS-PAGE and fluorography to assess lipoprotein content. The ability to produce Plp-40 was found to be conserved among certain P. multocida reference and clinical strains of different host origins including avian, human, porcine, bovine, feline, canine, ovine, and cervine, but not rabbit. Production of a 40-kDa lipoprotein was exhibited by all clinical isolates of Pasteurella aerogenes, Pasteurella pneumotropica, Actinobacillus suis, Actinobacillus suis-like organism, and Actinobacillus pleuropneumoniae examined, but not Pasteurella (Mannheimia) haemolytica, Actinobacillus lignieresii, or Haemophilus spp. These data suggest that, while not all Pasteurellaceae are able to produce a 40-kDa lipoprotein under the present experimental conditions, expression is somewhat conserved among diverse isolates of disparate host origins.

In vitro susceptibility of porcine respiratory pathogens to tilmicosin

Bacterial isolates obtained from swine with various clinical diseases were tested for susceptibility to tilmicosin by minimum inhibitory concentration (MIC) and Kirby-Bauer disk diffusion tests using National Committee on Clinical Laboratory Standards methodology. The tilmicosin MIC90 was < or =0.125 microg/ml for Erysiopelothrix rhusiopathiae, < or = 1 microg/ml for Haemophilus parasuis isolates, 8 microg/ml for Actinobacillus suis and Pasteurella multocida type A, 16 microg/ml for toxigenic and nontoxigenic P. multocida type D, 64 microg/ml for Bordetella bronchiseptica, and >128 microg/ml for Staphylococcus hyicus and Streptococcus suis. The results of disk diffusion testing matched well with the MIC results for each pathogen. This in vitro survey of tilmicosin activity against various swine isolates suggests that further clinical evaluation of tilmicosin in swine may be warranted for disease associated with E. rhusiopathiae, H. parasuis, and A. suis but not B. bronchiseptica, S. suis, or S. hyicus.

Antimicrobial susceptibility of Mycoplasma hyorhinis

A broth microdilution technique was used to determine the antimicrobial susceptibility of 15 field isolates of Mycoplasma hyorhinis to 10 antimicrobial agents, representative of different classes, and contrasting newer agents to existing ones. For the macrolides, the MIC(90) for tylosin and tilmicosin was 1 and 4 microg/ml, respectively, but was > or = 16 microg/ml for erythromycin. Tetracycline, lincomycin and enrofloxacin each had an MIC(90) of 2 microg/ml. The mycoplasma had similar levels of susceptibility to the aminoglycoside and aminocyclictol classes exhibiting an MIC(90) of 4 microg/ml for gentamicin and 2 microg/ml for spectinomycin. The isolates exhibited high MICs to trimethoprim/sulfamethoxazole with an MIC(90) > or = 16/304 microg/ml. In summary, M. hyorhinis isolates from the US had low MICs against a variety of antimicrobials tested, with the exception of erythromycin and trimethoprim/sulfamethoxazole.

Comparison of Pasteurella spp. simultaneously isolated from nasal and transtracheal swabs from cattle with clinical signs of bovine respiratory disease

Twenty-four matched pairs of isolates of Pasteurella haemolytica and three matched pairs of isolates of Pasteurella multocida were isolated by using a nasal swab and a transtracheal swab from individual calves with clinical signs of bovine respiratory disease. The identity of each matched pair was confirmed biochemically and serologically. The similarity of the isolates obtained from a nasal swab and from a transtracheal swab was compared by using ribotyping and antibiotic susceptibility analyses. Although the calves were sampled only once with a nasal and a transtracheal swab, when both samples were bacteriologically positive the nasal swab identified the same bacterial species as the transtracheal swab 96% of the time. The nasal swab isolate was genetically identical to the transtracheal isolate in 70% of the matched pairs. Six different ribotypes were observed for the P. haemolytica isolates, while only one ribotype was observed for the limited number of P. multocida isolates. Of the six P. haemolytica ribotypes, two ribotypes predominated. All the paired isolates displayed similar susceptibility to ceftiofur, erythromycin, tilmicosin, trimethoprim-sulfamethoxazole, and florfenicol, with some minor variations for ampicillin and spectinomycin. These results suggest that a nasal swab culture can be predictive of the bacterial pathogen within the lung when the isolates are from an acutely ill animal and can be used to determine antibiotic susceptibility.

Relationship between usage of antibiotics in food-producing animals and the appearance of antibiotic resistant bacteria

Many studies and meeting reports have suggested that the use of some antibiotics in food animals can compromise the treatment of some infectious diseases in humans. Although the studies and reports are timely and important, it is difficult to assess the relative value of the conclusions in relationship to the overall situation concerning antibiotic resistant foodborne bacteria because the data used in the analyses are often of disparate origin. The studies have attempted to establish a cause and effect relationship between the use ('consumption') of antibiotics in food animals and treatment failures in human disease on the basis of [1] antibiotic usage data; [2] in vitro determinations of antibiotic susceptibility of animal and human isolates, [3] results obtained from controlled animal experiments or [4] epidemiological data. Each approach has sought to associate bacterial antibiotic resistance data with it's own immediate focus area of investigation. However, a true assessment of the degree of contribution to human antibiotic resistance problems from animal use can only be facilitated by comprehensively organizing these different approaches into a concerted, coordinated effort. Concurrently, the implementation of a multinational programme aimed at monitoring antibiotic usage in food animals and resistance in specific bacteria associated with those animals should be instituted. In parallel with this endeavour is the implementation of new prudent use guidelines for antibiotic use by veterinarians. Through the use of science-based approaches like these, the development and spread of antibiotic resistant bacteria associated with food animals could be minimized and contained.

Intracellular accumulation, subcellular distribution, and efflux of tilmicosin in bovine mammary, blood, and lung cells

Tilmicosin is a semisynthetic macrolide antibiotic currently approved for veterinary use in cattle and swine to combat respiratory disease. Because the concentrations of tilmicosin are generally low in bovine serum, the interaction of tilmicosin with three types of bovine phagocytes (monocyte-macrophages, macrophages, and neutrophils from blood, lungs, and mammary gland, respectively) and mammary gland epithelial cells was evaluated to provide an understanding of potential clinical efficacy. After incubation with radiolabeled tilmicosin, uptake was determined and expressed as the ratio of the intracellular to the extracellular drug concentration. Accumulation of tilmicosin at 4 h of incubation by the alveolar macrophages (Cc/Ce 193) was 4 to 13 times more than that observed in monocyte-macrophages (Cc/Ce 43), neutrophils, (Cc/Ce 13), or mammary epithelial cells (Cc/Ce 20). Subcellular distribution showed that 70 to 80% of tilmicosin was localized in the lysosomes. Uptake in mammary gland cells was dependent on cell viability, temperature, and pH, but was not influenced by metabolic inhibitors or anaerobiosis. However, lipopolysaccharide exposure increased tilmicosin uptake by the bovine mammary macrophages and epithelial cells. When neutrophils and epithelial cells were incubated in the presence of tilmicosin and extracellular tilmicosin was then removed, 40% of the intracellular tilmicosin remained cell associated after 4 h of incubation (i.e., 60% effluxed), but only 25% remained in macrophages. These in vitro interactions of tilmicosin with bovine phagocytes and epithelial cells suggest an integral role in effecting clinical efficacy.

Comparison of tilmicosin and cephapirin as therapeutics for Staphylococcus aureus mastitis at dry-off

Forty-four cows (26 Jerseys and 18 Holsteins) that had at least 1 mammary quarter that was naturally (n = 12) or experimentally (n = 84) infected with Staphylococcus aureus were allotted to three treatment groups of approximately equal number at the end of lactation. Cows were dried off by abrupt cessation of milking, and dry cow therapy was administered as an intramammary infusion of cephapirin benzathine at 10 ml per quarter, an intramammary infusion of tilmicosin (solution containing 300 mg/ml) at 5 ml per quarter, or a subcutaneous injection of tilmicosin at 5 mg/kg of body weight on the day of drying off and another injection 4 d later. Mammary secretions were monitored during the dry period and postpartum for antimicrobial residues, intramammary infection (IMI) status, and somatic cell counts. Results demonstrated the following percentage cures for IMI caused by Staph. aureus at 28 d postcalving based on individual mammary quarters: cephapirin benzathine, 78.1%; tilmicosin infused, 74.2%; and tilmicosin injected, 9.1%. During the first 4 wk after drying off, the mean concentration of tilmicosin in mammary secretions from cows infused with the antibiotic remained approximately 10-fold higher than that in secretions from cows injected with the antibiotic (3.43 vs. 0.32 ppm), and, by the time of calving, concentrations for cows treated with both methods were below the dilution limit of the assay (< 0.1 ppm). Results demonstrated that intramammary infusion of tilmicosin was equally as effective as cephapirin benzathine in curing IMI caused by Staph. aureus at drying off; however, the subcutaneous injection of tilmicosin at the dose used was not effective as a dry cow therapeutic against Staph. aureus.

The determination of the cellular volume of avian, porcine and bovine phagocytes and bovine mammary epithelial cells and its relationship to uptake of tilmicosin

In order to compare the intracellular concentration of antimicrobial agents in phagocytic and nonphagocytic cells, the knowledge of their cell volume is essential. For the first time, the determination of the avian, porcine, and bovine polymorphonuclear neutrophils (PMN), monocyte-derived macrophages, macrophages, and bovine mammary epithelial cell volume was performed using [3H]-water and [14C]-carboxyinulin. The comparison of all the cells showed that the PMN have a size range between 3.58 and 4.04 microL per mg of protein, and are smaller than the monocyte-derived macrophages and mammary epithelial cells (4.32-5.01 microL per mg of protein). The macrophages show the largest size (5.84-6.57 microL per mg of protein). The cellular uptake of tilmicosin in these cells was then determined. The examination of the intracellular/extracellular concentration ratios (Ci/Ce) after 4 h of incubation with 10 mg/mL of [14C]-labelled tilmicosin revealed that tilmicosin was well accumulated and showed a ratio of 137, 169 and 193 in avian PMN, porcine PMN, and bovine alveolar macrophages, respectively. The cellular uptake data also demonstrated that tilmicosin accumulated in nonphagocytic bovine mammary epithelial cells. The importance of the use of the appropriate species and cell type specific cell volume values for calculations was exemplified by calculating the Ci/Ce of tilmicosin using cell volume data found in the literature for human and mouse cells. The subsequent comparison of these data with the Ci/Ce calculated with the actual cell volume appropriate for the species tested revealed an under evaluation of 3-13% in monocyte-macrophages, an over evaluation of 7-18%, 16-31% and 69% in PMN, macrophages, and epithelial cells, respectively. This study highlights the importance of the proper cell volume in order to determine the Ci/Ce. Moreover, the cell volumes determined here for avian, porcine and bovine cells should facilitate further in vitro and in vivo cellular studies by veterinary researchers.

Computed axial tomography of the porcine nasal cavity and a morphometric comparison of the nasal turbinates with other visualization techniques

A non-invasive imaging modality, computed tomography (CT), was used to visualize changes in nasal turbinates of anesthetized pigs over a 12-week observation period (pigs were 14 wk of age at study week 0). Normal, non-infected pigs were compared to pigs with mild challenge-induced atrophic rhinitis (AR) in order to detect subtle differences in morphology. To determine feasibility for time course studies in future experiments, morphometric quantitation at the level of the 2nd premolar (turbinate area ratio or TAR) in cross-section CT images at multiple timepoints was done. Additionally, at study termination, the TAR determined from CT images, magnetic resonance imaging (MRI), and wet tissue (WT), were compared to each other and to the standard subjective measure, visual scoring. There were no statistically significant differences between the control and AR groups at CT imaging dates of 0, 3, 6, 9, or 12 wk (P = 0.182). However, a statistically significant decrease in TAR measurements over time (P = 0.015) was observed in both groups, with lower mean values observed on Weeks 3 and 6 before rebounding to baseline values at study termination. At Week 12 (termination of the study), the TAR measurements derived from CT, MRI, and WT were not statistically different from one another (P = 0.220) and the treatment group-by-method interaction was not significant (P = 0.800). This provided evidence of equivalency of the techniques. Mean values for normal and infected groups were not significantly different based on either TAR imaging methods (P = 0.552) or visual scores (P = 0.088). Thus, the current study demonstrated that CT was an acceptable alternative imaging modality which could be used for quantitation of turbinate changes in snouts of live pigs to provide data comparable to tissue taken at necropsy. Computed tomographic imaging would allow non-invasive tracking of disease or treatment responses within individual animals over time. Morphometric analysis of the TAR was equivalent between the CT, MRI, and WT specimens.

Intracellular accumulation, subcellular distribution, and efflux of tilmicosin in chicken phagocytes

Tilmicosin is a semi-synthetic macrolide antibiotic, currently approved for veterinary use in cattle and swine respiratory disease, and is in development for use in poultry mycoplasma air sacculitis. In order to provide an understanding of clinical efficacy, the in vitro interaction of tilmicosin with three types of chicken phagocytes (MQ-NCSU macrophages, monocyte-macrophages, and heterophils) was evaluated. After incubation with radiolabeled tilmicosin, uptake was determined and expressed as the ratio of the cellular (Cc) to the extracellular (Ce) drug concentration (Cc:Ce). Tilmicosin was avidly accumulated by heterophils (Cc: Ce 138 at 4 h incubation vs 32 and 66, respectively, in MQ-NCSU and monocyte-macrophages) with 61 to 88% localized in the lysosomes. Uptake was dependent on cell viability, temperature, and pH, but was not influenced by metabolic inhibitors. However, phagocytosis of Pasteurella multocida and lipopolysaccharide exposure increased tilmicosin uptake by the chicken phagocytes. Upon removal of extracellular tilmicosin, 50% of the intracellular tilmicosin was effluxed within the first 30 min, but after 4 h of incubation in antibiotic-free medium, 30% remained cell-associated. Opsonized P. multocida significantly enhanced the release of tilmicosin from all three types of chicken phagocytes. Tilmicosin uptake was observed to increase lysosomal enzyme (acid phosphatase, lysozyme, avidin, and beta-glucuronidase) production. Finally, neutrophils were shown to transport and efflux bioactive tilmicosin in a test system measuring both neutrophil chemotaxis under agarose and a bioassay measuring inhibition of bacterial growth in the presence of antibiotic in agar. These in vitro observations of cellular pharmacology suggest a complex interaction between phagocytes and tilmicosin that contribute to clinical efficacy.

Intracellular accumulation, subcellular distribution and efflux of tilmicosin in swine phagocytes

Tilmicosin is a semi-synthetic macrolide antibiotic, currently approved for veterinary use in cattle and swine respiratory disease. As the concentrations of tilmicosin are generally low in swine lung tissue, the interaction of tilmicosin with three types of swine phagocytes (monocyte-macrophages, alveolar macrophages, and neutrophils) was evaluated to provide an understanding of clinical efficacy. After incubation with radiolabelled tilmicosin, uptake was determined and expressed as the ratio of the intracellular (Ci) to the extracellular (Ce) drug concentration (Ci/Ce). Tilmicosin was avidly accumulated by the swine phagocytes (Ci/Ce 48-69 at 4 h incubation) with 51 to 85% localized in the lysosomes. Uptake was dependent on cell viability, temperature and pH, but was not influenced by the metabolic inhibitors, sodium cyanide or potassium fluoride. However, lipopolysaccharide (LPS) exposure increased tilmicosin uptake by the swine phagocytes. In neutrophils, upon removal of extracellular tilmicosin, 60% of the intracellular tilmicosin was effluxed within the first 30 min, but after 4 h of incubation in drug-free medium, 25% remained cell-associated. In contrast, after 4 h of incubation in drug-free medium, 60% and 45% of tilmicosin remained cell-associated, within alveolar macrophages and monocyte-derived macrophages, respectively. Tilmicosin uptake was observed to increase lysosomal enzyme (acid phosphatase, lysozyme and beta-glucuronidase) production. Finally, neutrophils were shown to transport and efflux bioactive tilmicosin in a test system measuring both neutrophil chemotaxis under agarose and a bioassay measuring inhibition of bacterial growth in the presence of antibiotic in agar. These in vitro interactions of tilmicosin with swine phagocytes suggest an integral role in effecting clinical efficacy.

The effects of macrolides on the expression of bacterial virulence mechanisms

Macrolides are unique among the various classes of antimicrobial agents because of the manner in which they interact with pathogens and the host immune system to produce a clinical response. Conventional efficacy explanations require the serum or tissue concentration of the macrolide to exceed the MIC of the macrolide to account for bacteriostatic activity. However, at concentrations below the MIC, there are other effects on the expression of proteinaceous virulence factors which could prevent the pathogen from establishing or expanding an infection. The purpose of this review is to describe these effects and to provide an in-vivo scenario delineating the role of macrolides and leucocytes as they contribute to clinical efficacy.

Antimicrobial susceptibility of Moraxella bovis

The antimicrobial susceptibility of 55 isolates of Moraxella bovis to seven antibiotics was evaluated by broth microdilution procedures. The isolates had an MIC90 of < or = 1 mg/l to erythromycin, ceftiofur, and ampicillin; 4 mg/l to tilmicosin; 16 mg/l to tylosin and gentamicin; and had MIC90s of > or = 32 mg/l for oxytetracycline. The modal MIC values for these antibiotics were as follows: ampicillin, < 0.25 mg/l; ceftiofur, < or = 0.125 mg/l; tilmicosin, 2 mg/l; tylosin, 8 mg/l; erythromycin 1 mg/l; oxytetracycline, < or = 0.5 mg/l; and gentamicin, < or = 0.5 mg/l. This in vitro data showed most antibiotics have low MICs that are suggestive of clinical efficacy.

The effects of environmental conditions on the in vitro activity of selected antimicrobial agents against Escherichia coli

Various environmental conditions likely to be encountered at a nidus of infection were evaluated for their effect on selected classes of antimicrobial agents. The minimum inhibitory concentration (MIC) of several aminoglycosides (apramycin, kanamycin, gentamicin, tobramycin, amikacin), tetracycline, and chloramphenicol for five strains of E. coli were unchanged by temperature (35 degrees-39.5 degrees C), atmosphere (aerobic to anaerobic), pH > 7, NaCl concentration (up to 150 mM), zinc concentration (up to 50 mM), and manganese (up to 10 mM). However, the aminoglycoside MICs were increased up to fivefold at pH < 6.5. Magnesium and calcium ion concentrations >10 mM and ferric iron concentrations >/=10 mM increased aminoglycoside MICs from 3.66- to 8-fold. Tetracycline MICs were increased 1.2- to 6.5-fold when the concentration of magnesium or calcium was >/=10 mM. The results of this in vitro study might provide insight into the effects of local in vivo environmental conditions on several classes of antimicrobial agents.

In-vitro antimicrobial susceptibility of Clostridium perfringens from commercial turkey and broiler chicken origin

The minimum inhibitory concentrations (MIC) of eight antibiotics and two anticoccidial agents were determined for Clostridium perfringens strains isolated from 26 commercial broiler farms and 22 commercial turkey farms. Isolates were obtained from the intestines of birds on the farm or as the processing plant using standard culture and identification techniques. The microbroth dilution test was used to determine the MIC for each compound. Most isolates from chickens had MICs in the range of 2-16 mg/L for tilmicosin, tylosin and virginiamycin, whereas the MICs for avilamycin, avoparcin, monensin, narasin and penicillin were < or = 1 mg/L. Most strains from chickens had high MICs (> or = 64 mg/L) and appeared to be resistant to bacitracin and lincomycin. Most turkey isolates had MICs in the range of 2-16 mg/L for bacitracin, tilmicosin, tylosin and virginiamycin, with strains exhibiting MICs < or = 1 mg/L for avilamycin, avoparcin, monensin, narasin and penicillin. Several turkey isolates had MICs > or = 64 mg/L to lincomycin. No attempt was made to associate farm usage of a particular antibiotic to the antibiograms.

Serum haptoglobin concentration in growing swine after intranasal challenge with Bordetella bronchiseptica and toxigenic Pasteurella multocida type D

The acute phase reaction, in association with progressive atrophic rhinitis (AR), was monitored for 3 wk using serum haptoglobin (HPT) quantification in thirty-six, 15 kg swine after intranasal challenge with varying doses of Pasteurella multocida type D (toxigenic strain) and Bordetella bronchiseptica. The challenge doses were administered alone or in combination with pigs divided into 9 isolated treatment groups. Increasing doses of B. bronchiseptica were associated with lower serum HPT (P < 0.05), whereas increasing doses of P. multocida tended to increase serum HPT (0.05 < P < 0.10). Significant and positive correlation of mean HPT and AR score was found in these pigs; increased AR scores were associated with elevated mean HPT concentration (r = 0.41, P < 0.01). A significant interaction between P. multocida and B. bronchiseptica dose indicated that increasing the dose of B. bronchiseptica, for a fixed P. multocida dose, was associated with less AR (P < 0.05). The AR scores were greater in pigs given P. multocida, than B. bronchiseptica alone. These results indicate that a complex interaction between Pasteurella multocida and Bordetella bronchiseptica causes progressive atrophic rhinitis and alters serum HPT concentration in swine.

Minimum inhibitory concentration breakpoints and disk diffusion inhibitory zone interpretive criteria for tilmicosin susceptibility testing against Pasteurella spp. associated with bovine respiratory disease

Tilmicosin is a novel macrolide antibiotic developed for exclusive use in veterinary medicine. The first tilmicosin-containing product was approved to treat bovine respiratory disease associated with pasteurellae. The development of antimicrobial susceptibility testing guidelines for tilmicosin was predicated on the relationship of clinical efficacy studies that demonstrated a favorable therapeutic outcome, on pharmaco-kinetic data, and on in vitro test data, as recommended by the National Committee for Clinical Laboratory Standards (NCCLS). The NCCLS-approved breakpoints for the MIC dilution testing are resistant > or = 32 micrograms/ml, intermediate 16 micrograms/ml, and susceptible < or = 8 micrograms/ml. The zone of inhibition interpretive criteria for disk diffusion testing with a 15-micrograms disk are resistant < or = 10 mm, intermediate 11-13 mm, and susceptible > or = 14 mm.

The quantitation of turbinate atrophy in pigs to measure the severity of induced atrophic rhinitis

The two-fold purpose of this study was to establish a useful image analysis technique for quantitation of turbinate atrophy and to determine an optimum bacterial dose for inducing atrophic rhinitis (AR). Two morphometric analysis methods were compared to determine a turbinate area ratio (TAR) and a turbinate perimeter ratio (TPR); the ratios of turbinate area to total nostril area and of turbinate perimeter to total nostril perimeter, respectively. Our first image analysis method differed from Collins et al (1) in that we used direct image capture (digitalization) via a video camera and a Macintosh microcomputer, rather than photographs and a digitizer tablet. The tracing techniques were the same as those used by Collins et al. The second morphometric method was modified from the first by exclusion of dorsal turbinate when tracing the nostril area and directly tracing only the ventral turbinate to get a turbinate measurement without subtracting. Area and perimeter ratios, for both methods, were compared to conventional visual snout scores, ventral measurements, and to each other. The results of the two image analysis methods correlated well, both with each other and with the visual scores. Doses of Pasteurella multocida (Pm) at a constant level, and Bordetella bronchiseptica (Bb) at various concentrations, were administered to 36 Hampshire-Duroc F1 SPF pigs to determine the best dose and frequency for inducing AR. Although the dose selection may have been somewhat affected by the pre-existing presence of Bb, the optimal dose per naris in this study was 2 mL Bb at 10(7) cfu/mL combined with 2 mL Pm at 10(9) cfu/mL inoculum. The frequency of administration (1 x or 2 x) did not greatly affect results. Turbinate area ratio was the best tool for quantitating gross morphological turbinate changes associated with atrophic rhinitis in this study. Our simplified modification of Collins et al image analysis method (exclusion of dorsal turbinates and direct measurement of ventral turbinates) correlated well with visual scores, and, when compared to Collins et al method, required less data manipulation and labour.

Synthesis, antimicrobial activity and in vivo fluorine NMR of a hexafluorinated derivative of tilmicosin

A new fluorinated analog of tilmicosin was synthesized by the reductive amination of desmycosin with 3,5-bis(trifluoromethyl)piperidine. Despite an apparently small change in structure, the fluorinated analog had much less in vitro antimicrobial activity than tilmicosin and it failed to protect 3-day old chicks against a Pasteurella multocida challenge at 64 mg/kg sc. In a preliminary in vivo fluorine NMR experiment in a female Sprague-Dawley rat, a 19F NMR signal was detected in the liver one hour after ip administration of the fluorinated compound. Therefore, although this fluorinated derivative had less antimicrobial activity than tilmicosin, it may nevertheless provide a suitable model of tilmicosin for pharmacokinetic studies using in vivo fluorine NMR.

Proposed quality control guidelines for antimicrobial susceptibility tests using tilmicosin

Quality control guidelines for tilmicosin, a novel veterinary-use-only macrolide, were developed in a multi-laboratory study according to established National Committee for Clinical Laboratory Standards (NCCLS) procedures (M23-T2). Tilmicosin was incorporated into Sensititre plates for broth microdilution endpoint testing and into two lots of 15-micrograms disks for Kirby-Bauer agar disk diffusion testing. One common lot and five unique lots of Mueller-Hinton media were used. (Broth was cation adjusted, and agar was supplemented with 5% defibrinated sheep blood.) Bacteria used for reference strains included Pasteurella haemolytica 128K, Pasteurella multocida ATCC 43137, and Staphylococcus aureus ATCC 29213 (microdilution) and ATCC 25923 (disk). Replicate tests were conducted. Disk diffusion and broth microdilution quality control ranges are proposed.

Characterization of an Actinobacillus pleuropneumoniae seeder pig challenge-exposure model

Five strains of Actinobacillus pleuropneumoniae serotype 1 were used to intranasally infect 5 groups of pigs. Using each bacterial strain, infected pigs (termed seeder pigs) were commingled for 48 hours with 5 groups of noninfected test pigs, then were removed. Seeder and test pigs were maintained in isolation and were observed for 14 days. Seeder pigs had mortality that was threefold greater than that of test pigs (24% vs 8%). Rectal temperature in excess of 40.3 C was achieved for 84% of test pigs and 88% of seeder pigs. Neither of these 2 variables was statistically different between the 2 groups of pigs. Clinical impression scores > or = 2 (on a 0 to 3 scale) were three-fold (64% vs 20%) greater for seeder than for test pigs (P < 0.05). The total number of bacterial isolations or nonrecoverable isolates was tabulated for test and seeder pigs' lungs at necropsy, irrespective of the amount of lesions. The number of A pleuropneumoniae isolations was not statistically different between test and seeder pig populations. Recovery of Pasteurella multocida or other bacteria was greater from the seeder pigs (P < 0.05), whereas the number of non-recoverable isolates was greater from test pigs than from seeder pigs (P < 0.05). Assessment of lung lesions at necropsy by either visual estimation or on a weight basis were in agreement.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bentonite incorporated in a feed ration with tilmicosin in the prevention of induced Mycoplasma gallisepticum airsacculitis in broiler chickens

A factorial arrangement of tilmicosin and bentonite was evaluated for efficacy in broiler chickens infected with Mycoplasma gallisepticum and correlated to tilmicosin recovery in a feed assay method. Tilmicosin at 300-500 g/ton prevented development of airsacculitis. The addition of 2% bentonite to the ration caused tilmicosin at 300 g/ton to be ineffective in controlling air-sac lesions, whereas 400 and 500 g/ton were moderately effective. Six percent bentonite rendered tilmicosin completely ineffective at all dose levels. There was a direct correlation between the percentage of bentonite in the feed rations, the percentage of the tilmicosin recovered in the assay procedure, and the increased incidence of air-sac lesions.

The accumulation of bactericidal lipids in staphylococcal abscesses

Abscesses were generated in the peritoneal cavity of mice by the inoculation of 10(9) staphylococci. Abscess weight increased rapidly, reaching about 200 mg by the fourth day; for the next 60 days, abscess weight increased only slightly. The amount of total lipid increased during abscess development, attaining a peak level of about 19 mg per abscess at 7 days before decreasing. Almost all of this lipid resulted from the accumulation of neutral lipids. The small increases seen in the phospholipid and glycolipid fractions could be accounted for through the accumulation of host cellular elements in the abscess. Leucocytes containing cytoplasmic lipid droplets were first seen 4-12 h after infection and these cells were widely scattered around the periphery. During the next 2 days, the number of cells with lipid droplets increased markedly and lipid droplets were also found in the deeper portions of the abscesses. Although lipid droplets were found subsequently throughout the abscess, the greatest amounts always occurred in the leucocyte zone immediately proximal to the connective tissue capsule. During abscess development, the bactericidal activity also increased rapidly, reaching a maximum by the seventh day and declining thereafter.

Modification of bactericidal fatty acids by an enzyme of Staphylococcus aureus

Certain strains of Staphylococcus aureus produce an enzyme capable of inactivating the bactericidal fatty acids produced in staphylococcal abscesses by esterification to various alcohols. The enzyme, called FAME (fatty acid modifying enzyme), has a pH optimum between 5.5 and 6.0 and a temperature optimum of about 40 degrees C. Enzyme activity is not affected by edetic acid or by the presence or absence of sodium and potassium ions. Although FAME can utilise methanol, ethanol, 1-propanol, 2-propanol, 1-butanol or cholesterol as substrates, cholesterol appears to be the preferred substrate. FAME esterifies without being an esterase operating in reverse. Strains capable of producing the enzyme can synthesise it in trypticase soy broth and in a chemically defined medium, but not necessarily in equal amounts. FAME production is correlated with the ability of a strain to grow and survive within the tissues.

The production of bactericidal fatty acids from glycerides in staphylococcal abscesses

Staphylococcal abscesses contain two types of lipids which are bactericidal for Staphylococcus aureus. These include a group of long chain unsaturated free fatty acids and another as yet unidentified lipid with unique properties. When abscess homogenates are incubated with S. aureus culture filtrates, the amount of bactericidal activity is increased. This phenomenon is called activation. To determine the source of increased bactericidal activity during activation, individual types of lipid were isolated from abscess homogenates and examined for their ability to be activated. Activation was found to result from the release of long chain unsaturated fatty acids from glycerides, presumably by the action of staphylococcal lipase.

Antimicrobial susceptibility of Bordetella avium and Bordetella bronchiseptica isolates

Two veterinary pathogens, Bordetella bronchiseptica and Bordetella avium, were tested for their antimicrobial susceptibilities. Of the 20 antimicrobial agents tested, both species were consistently resistant to penicillin and cefuroxime but susceptible to mezlocillin, piperacillin, gentamicin, amikacin, and cefoperazone.

Association with phagocytic inhibition of anti-Pseudomonas aeruginosa immunoglobulin G antibody subclass levels in serum from patients with cystic fibrosis

Serum from cystic fibrosis patients colonized with Pseudomonas aeruginosa specifically inhibits phagocytosis of P. aeruginosa by alveolar macrophages. Serum was examined for P. aeruginosa lipopolysaccharide-specific immunoglobulin G (IgG) subclass levels (by enzyme-linked immunosorbent assay) and for the effect on macrophage phagocytosis (by radiolabeled P. aeruginosa uptake). Sera from cystic fibrosis patients with no known P. aeruginosa colonization history had negligible amounts of lipopolysaccharide-specific IgG and a mean phagocytic enhancement of 5%. The sera of normal volunteers also had negligible amounts of lipopolysaccharide-specific IgG. Serum from cystic fibrosis patients with P. aeruginosa respiratory tract infections had substantial titers (range, 1:20 to 1:1,280) of lipopolysaccharide-specific IgG2, IgG3, and IgG4 and a mean phagocytic inhibition of 56%. However, these patients had low or absent titers of lipopolysaccharide-specific IgG1. No consistent variation in the level of individual IgG subclasses in the sera of colonized patients was observed, as determined by radial immunodiffusion. The results suggest that during P. aeruginosa infection phagocytosis-inhibitory activity develops coincident with production of lipopolysaccharide-specific IgG subclasses.